1. Field of the Invention
The present invention relates to a liquid crystal display device and more particularly relates to a liquid crystal display device suitably used for portable information terminals (for example, PDAs), mobile phones, car-mounted liquid crystal displays, digital cameras, PCs, amusement equipment, TVs and the like.
2. Description of the Related Art
The information infrastructure is advancing day to day, and equipment such as mobile phones, PDAs, digital cameras, video cameras and car navigators has penetrated deeply into people's lives. Liquid crystal display (LCD) devices have been adopted in most of such equipment. With increase of the information amount handled with the main bodies of the equipment, LCD devices are requested to display a larger amount of information, and are demanded by the market for higher contrast, a wider viewing angle, higher brightness, multiple colors and higher definition.
A vertical alignment mode using a vertically aligned liquid crystal layer has increasingly received attention as a display mode enabling high contrast and a wide viewing angle. The vertically aligned liquid crystal layer is generally obtained using a vertical alignment film and a liquid crystal material having negative dielectric anisotropy.
For example, Japanese Laid-Open Patent Publication No. 6-301036 (Literature 1) discloses an LCD device in which an inclined electric field is generated around an opening formed in a counter electrode that faces a pixel electrode via a liquid crystal layer, so that liquid crystal molecules surrounding liquid crystal molecules existing in the opening, which are in the vertically aligned state, are aligned in inclined directions around the opening as the center, to thereby improve the visual angle characteristics.
However, in the device described in Literature 1, it is difficult to generate an inclined electric field over the entire region of each pixel. Therefore, each pixel has a region in which liquid crystal molecules delay in response to a voltage, and this causes a problem of occurrence of an afterimage phenomenon.
To solve the above problem, Japanese Laid-Open Patent Publication No. 2000-47217 (Literature 2) discloses an LCD device in which a plurality of openings are provided regularly in a pixel electrode or a counter electrode, to form a plurality of liquid crystal domains each having axisymmetric alignment in each pixel.
Japanese Laid-Open Patent Publication No. 2003-167253 (Literature 3) discloses a technology in which a plurality of projections are provided regularly in each pixel to stabilize the aligned state of liquid crystal domains having radially inclined alignment formed around the projections. This literature also discloses using an inclined electric field generated at openings formed in an electrode, together with the alignment regulating force of the projections, to regulate the alignment of liquid crystal molecules, and thus improve the display characteristics.
Japanese Laid-Open Patent Publication No. 2001-337332 (Literature 4) discloses a multi-domain vertically aligned LCD device in which wall spacers having inclined sides are provided to define the directions of tilt of liquid crystal molecules using the alignment regulating force of the inclined sides. This technology eliminates the necessity of performing an additional step for providing an alignment regulating structure and also can suppress a variation in inter-substrate spacing (thickness of the liquid crystal layer) even for large-screen devices.
In recent years, a type of LCD device providing high-quality display both outdoors and indoors has been proposed (see Japanese Patent Gazette No. 2955277 (Literature 5) and U.S. Pat. No. 6,195,140 (Literature 6), for example). In this type of LCD device, called a transflective LCD device, each pixel has a reflection region in which display is done in the reflection mode and a transmission region in which display is done in the transmission mode.
The currently available transflective LCD devices adopt an ECB mode, a TN mode and the like. Literature 3 described above also discloses adoption of the vertical alignment mode for, not only a transmissive LCD device, but also a transflective LCD device. Japanese Laid-Open Patent Publication No. 2002-350853 (Literature 7) discloses a technology in which in a transflective LCD device having a vertically aligned liquid crystal layer, the alignment (multi-axis alignment) of liquid crystal molecules is controlled with depressions formed on an insulating layer. The insulating layer is provided to make the thickness of the liquid crystal layer in a transmission region twice as large as that in a reflection region. According to this literature, the depressions are in the shape of a regular octagon, for example, and projections or slits (electrode openings) are formed at positions opposed to the depressions via the liquid crystal layer (see FIGS. 4 and 16 of Literature 7, for example).
The technology disclosed in Literature 2 or 3 has the following problems. Projections or openings are provided in each pixel to form a plurality of liquid crystal domains in the pixel (that is, divide the pixel into domains), to thereby strengthen the alignment regulating force on liquid crystal molecules. According to examinations conducted by the inventors of the present invention, however, to obtain sufficient alignment regulating force, it is necessary to form an alignment control structure such as projections and openings on both surfaces of the liquid crystal layer (on the surfaces of the pair of substrates opposed each other facing the liquid crystal layer), and this complicates the fabrication process. Moreover, the effective aperture ratio of a pixel having such an alignment regulating structure therein may decrease, and also the contrast ratio may decrease due to light leakage occurring in the peripheries of the projections in the pixel. In the case of providing the alignment regulating structure on both substrates, the substrate alignment margin must be taken into consideration. Therefore, the decrease in effective aperture ratio and/or the decrease in contrast ratio will become further conspicuous.
In the technology disclosed in Literature 4, a plurality of liquid crystal domains are formed in each pixel using wall spacers (the alignment direction of liquid crystal molecules is uniform in one domain and is different among different domains). Therefore, the wall spacers must be formed inside each pixel, and this causes decrease in effective aperture ratio and/or decrease in contrast ratio.
In the technology disclosed in Literature 7, it is necessary to provide projections or electrode openings at positions opposed to the depressions formed for control of the multi-axis alignment. This technology therefore has the same problems as those described above.
In any of the literature described above, openings are formed in the display electrodes so that the electroclinic alignment of liquid crystal molecules is defined with the effect of an electric field generated with application of a predetermined voltage. In this relation, when the liquid crystal panel is pressed, the aligned state of liquid crystal molecules disturbed with the pressing in the pressed portion tends to be fixed with the electric field defined with the electrode openings, and this may result in occurrence of display roughness and degradation in display quality.
In view of the above, an object of the present invention is providing a vertically aligned liquid crystal display device that can sufficiently stabilize the alignment of liquid crystal molecules with a comparatively simple construction and can provide display quality equal to or higher than that conventionally obtained.
Another object of the present invention is providing a liquid crystal display device having a plurality of axisymmetrically aligned domains in each pixel, in which axisymmetric alignment disturbed when the display screen is pressed, for example, can be effectively recovered and thus display failure such as roughness can be reduced to present high display quality.